I'm not sure about the "hardest." I think it's hard to grow a channel in general... But I do think some content takes a lot longer to make than others.
I think animation, short films, and actual music videos of original songs probably take the longest time to make. I can only really relate to the music videos... A lot of people tend to forget that artists need to create and edit the music for their music video, too. Sometimes it takes weeks or months to write and record a song. Whenever you hear a song that you think sounds cool, just know that the artist probably sang it 2000 times (not exaggerating...) and edited hundreds of takes together to make it sound that way... The actual video part doesn't really take that much time to do in comparison to the audio, but it's an extra step. For me, the most time consuming part is making musical backing tracks. The rest is usually pretty quick. I should probably spend more time on the video step, but I don't have the time and energy for it.
I think the fastest videos to make would probably be let's play, tutorials, vlogs - anything that just requires a person, a setting, and talking. I script and practice my tutorials before I film them, and a video is usually done in an hour or two. That's much easier than dealing with audio and backing tracks! ^_^
YouTube seems to reward those who can produce content on a frequent and regular basis, so trying to produce more time consuming content on a frequent basis becomes more taxing than producing less time consuming content on a frequent basis. I think there is less competition for time consuming content, but I'm not convinced that aspect outweighs the amount of time needed to create the content and YouTube's rewarding algorithms. I think YouTube has identified some of these issues, and I'm hopeful that they will add something in the future to help creators who cannot easily produce frequent content.... ^^
